Both professional and amateur golfers use multi-piece, solid golf balls today. Basically, a two-piece solid golf ball includes a solid inner core protected by an outer cover. The inner core is made of a natural or synthetic rubber such as polybutadiene, styrene butadiene, or polyisoprene. The cover surrounds the inner core and may be made of a variety of materials including ethylene acid copolymer ionomers, polyamides, polyesters, polyurethanes, and polyureas.
Three-piece, four-piece, and even five-piece balls have become more popular over the years. More golfers are playing with these multi-piece balls for several reasons including new manufacturing technologies, lower material costs, and desirable ball playing performance properties. Many golf balls used today have multi-layered cores comprising an inner core and at least one surrounding outer core layer. For example, the inner core may be made of a relatively soft and resilient material, while the outer core may be made of a harder and more rigid material. The “dual-core” sub-assembly is encapsulated by a single or multi-layered cover to provide a final ball assembly. Different materials are used in these golf ball constructions to impart specific properties and playing features to the ball.
For instance, in recent years, there has been high interest in using polyurethane compositions to make golf ball covers. Generally, polyurethane compositions contain urethane linkages formed by reacting an isocyanate group (—N═C═O) with a hydroxyl group (OH). Polyurethanes are produced by the reaction of a multi-functional isocyanate with a polyol in the presence of a catalyst and other additives. The chain length of the polyurethane prepolymer is extended by reacting it with hydroxyl-terminated and amine curing agents.
In Sullivan et al., U.S. Pat. No. 5,971,870, thermoplastic or thermosetting polyurethanes and ionomers are described as being suitable materials for making outer cover and any inner cover layer. The cover layers can be formed over the cores by injection-molding, compression molding, casting or other conventional molding techniques. Preferably, each cover layer is separately formed. In one embodiment, the inner cover layer is first injection molded over the core in a cavity mold, subsequently any intermediate cover layers are injection molded over the inner cover layer in a cavity mold, and finally the outer cover layer is injection molded over the intermediate cover layers in a dimpled cavity mold.
In Sullivan et al., U.S. Pat. No. 7,131,915, the outer cover can be made from a polyurethane composition and various aliphatic and aromatic diisocyanates are described as being suitable for making the polyurethanes. Depending on the type of curing agent used, the polyurethane composition may be thermoplastic or thermoset in nature. Sullivan '915 further discloses that compositions for the intermediate cover layer and inner cover layer may be selected from the same class of materials as used for the outer cover layer. In other embodiments, ionomers such as HNPs, can be used to form the intermediate and inner cover layers. The castable, reactive liquid used to form the urethane elastomer material can be applied over the core using a variety of techniques such as spraying, dipping, spin coating, or flow coating methods.
Golf ball manufacturers often prefer thermoset polyurethane covers over thermoplastic polyurethane covers due to the excellent mechanical strength, impact durability, and cut and scuff (groove shear)-resistance of the former compared with the latter. In thermoset polyurethanes, a high degree of covalent cross-linking occurs, creating chemical bonds that are irreversibly set when the material is cured and which cannot be broken when exposed to heat. In contrast, thermoplastic polyurethanes have different bonds that can be reversibly broken by increasing temperature such as during molding or extrusion. Therefore, thermoplastic polyurethane cover layers aren't typically used without first treating the materials in some manner in an attempt to improve mechanical strength, impact durability, and cut and scuff (groove shear)-resistance.
Regardless, it is sometimes desirable to meanwhile create a hardness gradient in the cover layer in order to achieve desired golf ball characteristics including unique combinations of desired resilience, durability, compression, “feel,” and spin. In this regard, having a cover surface that is harder than the inner surface is one such desirable construction. For example, U.S. Pat. Nos. 8,523,708 and 8,747,254 of Sullivan et al. discloses golf balls having thermoset or thermoplastic polyurethane covers wherein the cover surface is hardened by treating the cover surface with and incorporating therein a fatty acid and/or fatty acid salt composition.
The fatty acid and/or fatty acid salt composition penetrates and embeds the cover surface and includes not only fatty acids and/or fatty acid salts but hardening ingredients as well such as zinc methacrylate, zinc dimethacylate, a thermoplastic resin and/or peroxide since fatty acids/fatty acid salts typically soften rather than harden materials. The fatty acid and/or fatty acid salt composition includes the hardening/stiffening material in an amount of from about 1 wt % to about 50 wt %. The weight % of the hardening/stiffening material in the fatty acid and/or fatty acid salt composition may be modified to achieve a cover layer outer surface hardness α that is greater than, less than, or even substantially similar to the cover layer inner surface hardness β, depending on the golf ball characteristic being targeted. Sullivan et al. teaches that the fatty acid and/or fatty acid salt compliments the stiffening agent by embedding itself between polymer chains, spacing the chains apart, and thereby increasing free volume in the material for penetration.
However, given that fatty acids/fatty acid salts generally soften rather than harden materials, it would be beneficial to develop golf balls wherein a thermoset polyurethane cover layer surface can be hardened with ingredients which simultaneously penetrate and harden the thermoset polyurethane cover surface without the need for fatty acids/fatty acid salts and without meanwhile causing the cover surface to become unduly brittle.
Such golf balls, if meanwhile also producible cost effectively within existing golf ball manufacturing processes and without sacrificing the mechanical strength, impact durability, and cut and scuff resistance of conventional thermoset polyurethane cover layers, would be particularly desirable. The golf balls of the invention and methods of making same address and solve this need.